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Acoustic Academy

Glossary of Acoustic Concepts & Parameters


    • A-weighting is a curve, or filter, that shapes decibels measured in a certain environment by taking in account the loudness with which we humans perceive sound throughout the audible frequency range – 20 Hz to 20kHz. Our hearing sensitivity varies depending on the different ranges of frequency (being more susceptible to sounds in the mid frequency range of 1kHz to 5kHz), therefore, each of them has an adequate correction to account for that. A-weighted measures are represented as dB(A).

    • Ambient Noise: is referred to as the continuous sound event of a certain environment, encapsulating both the quietest and the loudest ones, as well as sporadic events in between. It is often mathematically represented as the L(A)eq,t value.

    • Anechoic Chamber: space design isolated from the outside, and with fully absorbent walls that retain 100% of the sound waves that arrive to them. A person within an anechoic chamber only hears sound directly coming towards them, without any reflection. This makes it easier to perceive very quiet sounds too, such as components of the human body moving or flowing over time, like heart beats. A Semi- or Hemi- anechoic chamber includes a single solid surface, typically for the purpose of mounting heavy equipment, such as cars or machinery, in which is not possible in a fully anechoic chamber or to provide more representative conditions, i.e. a car will always be on a road reflective road surface.

    • Background Noise: also known as ‘Residual noise’, refers the quietest periods, where the noise environment is absent of noise nuances or intermittent events. It is mathematically represented as the L(A)90,t value.

    • Ctr: correction applied to certain acoustic parameters in order to account for the effect of low frequency noise, i.e. a basic double-glazed windows have an Rw of 31, however, when the correction is applied they have 27 Rw + Ctr. It is to be understood that even if the correction appears represented as a sum, low frequency noise weakens the power of an element to insulate sound, therefore, the correction is negative and the resulting value lower.

    • Decibel: is the unit used for measuring sound (and noise) levels. Its scale is logarithmic, which in practical terms means that the relation between values depends on the range where the values themselves are within the scale. It is generally accepted that under normal conditions humans are capable of detecting changes in steady noise levels of 3 dB, whilst a change of 10 dB is perceived as a doubling or halving of the noise level.  An indication of the range of noise levels commonly found in the environment is given below in Figure 1.

      Figure 1. Typical Noise Levels

    • Flanking: sound that transmits indirectly through spaces, either over or around the separating elements, i.e. pipework, ducting or electricity cables.

    • Frequency: measured in Hertz (Hz), represents the number of times a wave fluctuates per second. When talking about sound, frequency is directly proportional to the pitch we hear – the higher the frequency, the higher the pitch.

    • Frequency Bands: human ears perceive sounds  between 20Hz and 20kHz. In order to categorise/group such a wide range of frequencies, bands are introduced. This is key to understand the effects of a sound on the environment it is in, as well as how we react to it, and its characteristics.

    • L(A)10,t: a-weighted (A) noise level exceeded for 10% of the measurement period (t). It provides a measurement of the louder periods when noise events happen, dismissing the lower ones.

    • L(A)90,t: this is the a-weighted (A) noise level exceeded for 90% of the measurement period (t). It provides a measurement of the quieter ‘lull’ periods in between noise events and it is often referred to as the background sound  level.

    • L(A)eq,t: is the continuous a-weighted (A) time-average sound pressure level for a specific duration (t). This is commonly used to measure all sound sources in the environment and can be referred to as ambient noise.

    • L(A)max,F,t: is the maximum a-weighted (A) sound pressure level measured in a given time period (t) with the sound level meter set to ‘fast’ response (F). When the sound level meter is set to ‘Fast’ mode, the diaphragm of the device responds to alterations in level  within the range of 125 ms, which is indeed very reactive, therefore, manages to measure even the most impulsive sound sources.

    • Loudness: is a measure of human response to sound, related to the subjective impression of its magnitude. It not only links the intensity of the source, but to frequency as well, as the human ear responds differently to the various ranges and is most sensitive to sounds in the mid frequency range of 1 kHz to 5 kHz.

    • Resonance: happens when a mechanical or electrical system naturally vibrates at a certain rate, and therefore projects a specific focused frequency, which can vary in intensity depending on several factors, such as the power requirements or the shape of the system.

    • Reverberation: is a phenomenon that happens as a consequence of sound reflecting against the difference surfaces of an environment and is perceived as the continuity of sound even after the source has stopped producing it. Depending on how reflective or absorbent the surfaces are, the reverberation time can vary significantly in a space. 

    • Reverberation Time (RT): is the period that it takes for the reflections of a sound source to lower down a certain number of decibels. Generally, either 20 dB (RT20), 30 dB (RT30) or 60 dB (RT60).  Different spaces require different reverberation times, adequate to their fundamental purpose. For instance, long reverberation times are positive for concert halls, as they allow everybody (no matter where they are located within the venue), to perceive a lively and present performance, by hugely projecting sound throughout. On the other hand, long reverberation times are negative on school classes, as speech intelligibility decreases immensely, therefore, not allowing pupils to understand properly what the teacher is explaining and creating further problems.

    • Rw: sound reduction index as tested in lab – amount of sound reduction tested in an environment with no reflections, commonly known as ‘acoustic laboratory’ or ‘anechoic chambers’.

    • Sound Insulation: similarly to the capacity of certain elements to reduce the amount of heat or cold that transfers through spaces, sound insulation describes the same capacity, however applied to sound and noise transmission/mitigation.

    • Sound Power Level (SWL): is the continuous amount of sound energy emitted by a source, without taking in account the environment in which it resides.

    • Sound Pressure Level (SPL): it could be understood as the ‘consequence’ of sound power level in an environment. Sound pressure level takes in account factors such as the medium through which sound travels, the distance to the listener, and how reverberant the environment or space is, amongst others.

    Impact Sound Insulation

    • Ln,f,w: flanking sound reduction performance as tested on site – amount of flanking sound reduction of a material as tested in a reflective environment, setup in an overall floor assembly.

    • Ln,w: sound transfer via a floor assembly as tested on site, measuring the sound level in the receiving room – This measurement uses a ‘tapping’ machine to produce known level of tapping on a floor assembly and then measures the sound level in the adjoining room. The impact sound insulation measures the sound level audible in the adjoining room when the ‘tapping’ machine is in operation. Therefore, the lower the impact sound insulation value the better the performance.

    • ΔLw (Delta Lw): impact sound reduction benefit of a specific material as tested in lab – amount of impact sound reduction benefit when a material or product is added to a floor assembly. The higher the ΔLw value the better the performance. This metric is the most appropriate term for individual flooring elements, such as a carpet or wood flooring with acoustic backing, as it doesn’t eliminate the acoustic performance of the overall floor construction.

    Airborne Sound Insulation

    • Dn,f,w: flanking sound reduction index as tested on site – amount of sound reduction provided focused on flanking sound as tested in a reflective environment.

    • Dn,w: sound reduction index as tested on site– amount of sound reduction a separating partition (such as a wall) provides as tested in a reflective environment.

    • STC: sound reduction index as tested in lab – American version of Rw, which is European. STC values don’t necessarily translate to Rw ones, as they use different frequencies to specify products.

    Sound Absorption

    • A: denotes the absorption area, which is the absorption coefficient of a material multiplied by its surface area. I.e. if a carpet has a coefficient of 0.3 and a surface area of 20m2 then the element’s A = 6.

    • Classes A to E: materials are rated following ranges of absorption capacity, meaning, Class A products will be significantly more absorbent than Class E products. Class A products have an αw of higher than 0.9.

                                        Figure 2. Sound Absorption Classes Performance

    • αw (Absorption coefficient): value between 0 and 1, representing the quantity of absorption of a material. 1 being completely absorbent (not letting any sound reflect or through), and 0 being completely reflective. The w denotes a single number ‘weighted’ value summarising the performance across all sound frequencies.


    • d (Diffusion coefficient): its value ranges between 0 and 1. It represents the level of uniformity with which sound is reflected against a material, where a product with a coefficient value of 1 scatters sound completely uniformly, and 0 completely randomly.

    Acoustic Privacy

    • Privacy Factor: a function of the sound insulation performance (typically Dn,w) added to the indoor ambient noise level (dB LAeq). For example, if the sound insulation performance between two meeting rooms is 35 dB Dn,w and the ambient noise level in the most sensitive room is 40 dB LAeq, the resulting Privacy Factor is 75. Generally, a Privacy Factor below ‘75’ would result in speech being ‘clearly audible and intelligible’ in the adjacent room. A Privacy Factor above ‘80’ is considered good practice to avoid speech being intelligible between rooms, and above ‘85’ is advised to avoid any speech being audible.

    Ventilation Systems

    • IL (Insertion Loss): refers to the difference in level before and after a noise reducing system has been installed.

    • NR (Noise Rating curve): a standardised method of producing a single value for a background sound frequency spectrum. Typically, it is used in specifying maximum allowable sound conditions for room types in the design process. NR has no direct relation to dB(A), but as a general rule of thumb NR ≈ dB(A) - 6. 

    Vibration Isolation

    • Acceleration: the measure of the rate of change in material displacement that is caused by a source of vibration (or “excitement"). Typically, the acceleration is used to calculate the resulting impact of the vibration in terms of decibels.

    • Peak Particle Velocity: the maximum rate of displacement for a material set in motion by a source of vibration. Typically this unit is used to assess the level of potential building structure damage.

    • Vibration Dose Value: is the measure of cumulative vibration over a certain period, resulting in a value that indicates the threshold of human perception, i.e. to indicate annoyance due to a vibration source.


    • Values of sound reduction tested ‘on site’ (reflective environment) have a 5 to 10 dB difference against values tested on a ‘laboratory’ (non-reflective environment), as they consider on site weaknesses, i.e. interaction with  factors like the volume and the reverberation of the space where sound is being transmitted to.

    Acoustic Terminology : Noise Units

    • There is a ten million to one ratio between the threshold of hearing and the highest tolerable sound pressure.  Noise is therefore measured using a logarithmic scale, to account for this wide range, called the decibel (dB). Noise is defined as unwanted sound and the range of audible sound varies from around 0 dB to 140 dB.

      The human ear is capable of detecting sound over a range of frequencies from around 20 Hz to 20 kHz, however its response varies depending on the frequency and is most sensitive to sounds in the mid frequency range of 1 kHz to 5 kHz.  Instrumentation used to measure noise is therefore weighted across the frequency bands to represent the sensitivity of the ear. This is called ‘A weighting’ and is represented as dB(A).

      It is generally accepted that under normal conditions humans are capable of detecting changes in steady noise levels of 3 dB, whilst a change of 10 dB is perceived as a doubling or halving of the noise level.  An indication of the range of noise levels commonly found in the environment is given below.

      A number of different indices are used to describe the fluctuations in noise level over certain time periods.  The main indices include:


      This is the noise level exceeded for 90% of the measurement period and provides a measurement of the quieter ‘lull’ periods in between noise events.  It is often referred to as the background noise level.


      This is the “equivalent continuous A weighted sound pressure level” and is the level of a notional steady sound which has the same acoustic energy as the fluctuating sound over a specified time period.  It is often used for measuring all sources of noise in the environment, which can be referred to as the ambient noise.


      This is the maximum sound pressure level measured in a given time period with the sound level meter set to ‘fast’ response.

      Reference is often made to acoustic measurements being undertaken in ‘free-field’ or ‘façade’ locations.  Free-field measurements represent a location away from vertical reflecting surfaces, normally by at least 3.5 metres.  A façade measurement is undertaken or calculated to a position 1 metre from an external façade and a correction of up to 3 dB can be applied to account for the sound reflected from the façade.  This latter position is often used when assessing the impact of external noise affecting residents inside properties.